Friday, 15 April 2016

Airspy, Spectrumspy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer?

Airspy, SpectrumSpy, noise source and UHF cavity filter characteristics; a low cost spectrum analyzer?


A basic spectrum analyzer/tracking generator for less than $250? Yes. Can it be used to do a demanding task like tuning a UHF cavity filter from a repeater? Seems so.

The "proof of concept", spectrum analyzer software, SpectrumSpy, can be used with the Airspy SDR and a noise source to show the characteristics of a pass-reject UHF cavity filter.

SpectrumSpy and Airspy

SpectrumSpy, "proof of concept", spectrum analyzer software is a new addition to the SDR# download for use with the Airspy SDR. It has the potential for a new direction with low cost SDRs, spectrum analyzers.  Spectrum analyzers are expensive; $1500 then skyward. SpectrumSpy: (separate executable in SDR# folder).

Airspy has a 24 – 1800 MHz native RX range, but down to DC with the SpyVerter option. $199  and US$59

An earlier post is of SpectrumSpy used as a spectrum analyser:

However, a spectrum analyzer needs a tracking generator to be really useful and to test radio filters. A noise source can be used to achieve much the same purpose.

Noise source

A low cost  Zenar diode based noise source is available for about $20. It uses three Mini-Circuits ERA-5+ wideband amplifiers (DC-4 GHz) to get the noise to a usable level.

I did a quick check of the white noise with SpectrumSpy right up to 1 GHz. The level dropped a little with frequency, but that could be either the noise source or Airspy. and many others.

Noise source output

Cavity filter characteristics 

A cavity filter is part of a duplexer that allows a radio repeater to simultaneously transmit and receive with the same antenna, an amazing feat in itself and one of my other interests. Duplexers are fairly complex in design and setup.

Pass/reject is one type of cavity filter that passes the receive signal on one frequency, but rejects the re-transmit of the repeater on another frequency with a notch for the receive side of the duplexer and the opposite for the transmit side.

An expensive spectrum analyzer/tracking generator is needed to adjust the pass and reject frequencies and to minimize losses.

However, SpectrumSpy with a noise source does a pretty good job. The shape, frequencies and depth of notch (>30 dB) are about right in this very preliminary test.

Characteristics of single cavity filter using SpectrumSpy and noise source

Spectrum analyzer/tracking generator plot of similar but different filter.

Typical characteristics from half a duplexer (two cavities) with my old HP spectrum analyzer and tracking generator ($12K in 1990s and about $1000 secondhand now). (I will do it for the same filter another time and edit.) It has different tuning to the one in this post, hence the mirrored shape.


Airspy with SpectrumSpy does a good job in this demanding task. It probably could be used to tune a duplexer; amazing for the very low cost.

Two immediate issues however. First, SpectrumSpy is not calibrated or necessarily linear. Second, it does not have the bells and whistles of a spectrum analyzer such as digital analysis and data, or amplitude/frequency markers.

SpectrumSpy is proof of concept of an SDR used as a spectrum analyzer. This is a very good additional application for low cost SDRs, normally used as receivers. Similar software could be developed for other SDRs, as has been done, but less successfully, for the RTL-SDRs.

Thursday, 14 April 2016

Low cost spectrum analyser/ scanner with AirSpy and RTLSDR

Low cost spectrum analyser/scanner software for the Airspy and RTL-SDR


It is not often I am amazed at new technology, especially for free, but the Spectrum Spy software, a spectrum analyser/scanner for the Airspy SDR, impressed me. It is a poor man's spectrum analyser.

However, it is preceded by at least two spectrum scanners for the RTL-SDR hardware; rtl_power and RTLSDR Scanner.

This post will compare the two devices and three software packages, scanning the entire FM band and the 100 MHz of the local TV band.

The software

The three programs all run under Windows, Windows 10 in my case. All three installed and ran without much difficulty.

Spectrum Spy is part of the SDR# software package. It is a separate program to SDR#, but in the same folder. Spectrum Spy has a spectrum and a waterfall. It updates every few milliseconds, depending on the span. Spectrum Spy only works with the US$199 Airspy. I have a V1 Airspy.

RTLSDR-Scanner is a stand-alone program; Use the "setup" version and it will download all the code it needs to install. No extra files are needed for the RTLSDR, provided its driver is installed. RTLSDR-Scanner is a single pass with spectrum but no waterfall.

Rtl_power is a small program that has both spectrum and waterfall. Some additional RTLSDR files are needed to be copied to the program directory.

The hardware: apples and oranges

The Airspy and RTL-SDR both use the same tuner, but after that the Airspy is much more sophisticated. A significant difference is the frequency span, 10 MHz for Airspay and about 1.2 MHz for the RTL-SDR. The span makes a big difference to the scan rate of the scanner/analyser programs.

The question is whether the $20 RTL-SDR is useful as a scanner, compared to the Airspy.

FM band

A handy source of signals is the local FM band. The screen shots show how the three programs perform.

Spectrum Spy performs very well, showing a 20 MHz span. It shows both spectrum and waterfall. The waterfall is very useful for intermittent signals.

RTLSDR Scanner does a reasonable and useful job. The spectrum is comparable with Spectrum Spy. However, it only does a single scan and takes 20 seconds of so for the scan.

RTL_power had an indifferent result. The spectrum is quite different to the other two programs. While it does a continuous scan, the waterfall does not align with the spectrum.

TV band

The local DVB-T TV is in a 100 MHz band from 600 MHz. Displaying the TV stations is quite a test for a spectrum analyser.

The Spectrum Spy does a very good display in this demanding task.

The RTLSDR Scanner similarly does well in displaying the spectrum, although it takes quite some time to do the scan.

RTL_power does better with the TV than the FM, but is still not great.


The Spectrum Spy program with the Airspy hardware does an awesome job comparable to some spectrum analysers, for low cost,

It would be ideal for doing repeater site surveys, especially with the waterfall as well as the spectrum.

RTLSDR Scanner could be used in a similar manner to Spectrum Spy, just taking longer with a single scan.